This invention involves the interrelationship between the redox condition of melting glass and the solubility of certain compounds in the melt. More specifically, this invention deals with controlling this interrelationship so as to reduce emissions from the glass melting operation, to assure adequate fining of the glass, or to produce certain colored glasses.
Commercial production of glass conventionally involves feeding raw glass batch materials into an opening at one end of an elongated melting furnace while withdrawing melted glass through an opening at the opposite end of the furnace. When exposed to the high temperature conditions within the melting furnace, the raw materials undergo chemical reactions and dissolution which normally take place within the first portion of a continuous glass melting furnace. The remainder of the furnace is devoted to "fining" (or "refining") and conditioning the glass melt. The process of fining is the removal of gaseous products of reaction or other entrained gases from the melt by providing conditions which cause the gases to be driven from the molten glass. High temperature is one of the conditions that enhances fining, and for that reason the heat sources in a melting furnace are often arranged to provide the peak temperature downstream from the initial melting zone. Failure of the glass to be adequately fined can result in the undesirable presence of bubbles or "seeds" in the product glass.
Glass makers conventionally include in the mixture of batch materials being fed to a melting furnace at least one compound that is intended to serve as a fining aid or agent. A fining agent is characterized by its tendency to dissociate at relatively high temperatures, whereby it dissociates and forms substantial volumes of gas into which undesired species of gas diffuse and thereby more readily escape from the melt. Compounds of sulfur constitute the most common refining agents, particularly sodium sulfate (salt cake) and calcium sulfate (gypsum). Other sulfates, as well as sulfides and sulfites have also been suggested as fining agents. Injecting SO.sub.2 gas directly into the melting glass is proposed in U.S. Pat. No. 3,375,095 (Poole). Regardless of the form in which the sulfur is introduced into the melt, most of the sulfur is converted to sulfate, sometimes expressed equivalently as SO.sub.3 concentration, in which form a portion remains dissolved in the glass and the remainder dissociates, forming SO.sub.2 and O.sub.2. Carbonaceous material, usually powdered coal, is sometimes included in the batch mixture so as to increase the amount of sulfur that dissociates from the melting glass with the intention of enhancing the fining effect.
Ideally, the fining agent would dissociate when it is desired to begin the fining process, i.e., after the initial melting of the batch materials, as the melt enters the zone of peak temperature. Unfortunately, fining agents often begin dissociating as soon as they are exposed to the heat of the furnace and the reducing conditions produced by the presence of reducing agents such as carbon that are included in the batch mixtures. As a result, there is premature dissociation of the refining agent before the glass enters the fining zone. In order to provide sufficient fining agent for the downstream fining process, it has then been necessary to include excess fining agent in the batch mixture to compensate for the losses in the early melting stages. As a result, considerably more of products of the dissociation of the fining agent enter the exhaust gas stream from the furnace than is required for the fining process itself. The emissions from most fining agents are considered to be environmentally detrimental, and costly exhaust gas treatment is often required to avoid excessive discharge of these emissions to the atmosphere. Accordingly, it would be desirable to reduce the amount of refining agent required One approach to reducing the amounts of fining agent used is disclosed in U.S. Pat. No. 4,138,235 (Turner), but further reductions would be desirable.
Some colored glasses require that the reduction/oxidation ("redox") state of the melt be controlled so as to be relatively reducing so that certain multivalent colorant ions are induced to be present in the glass in their lower valence state. Iron is frequently used as a colorant in glass, and it is known that melting the glass under reducing conditions enhances the amount of the iron that is in the ferrous state rather than the ferric state, with the result that the glass is more absorbing in the infrared range of the spectrum. This is very desirable for glass that is intended to reduce the heat gain into enclosures such as buildings or automobiles. One prior art attempt to increase the infrared absorption of glass by increasing the amount of ferrous iron entailed including larger total amounts of iron in the glass. This approach is limited by the difficulty of melting glass with large amounts of iron due to lowered heat transfer. Using a moderate amount of iron and attempting to drive more of it to the ferrous state by providing a reducing environment in the melter by including a reducing agent (such as powdered coal or other carbon source) in the batch materials or by firing the furnace with an excess of fuel has also been less than successful because of inadequate fining. Production of relatively reduced glass by introduction of reductant with the batch mixture can lead to premature dissociation of the fining agents so that amounts remain at the fining zone, with the consequence that fining may be inadequate. Specialized melting and fining means as disclosed in U.S. Pat. No. 4,792,536 (Pecoraro and Shelestak) can avoid these problems, but it would be desirable to be able to produce reduced glasses with the large capacity of conventional glass melting furnaces presently available.
U.S. Pat. No. 2,330,324 (Adams) discloses a process of first bubbling carbon monoxide (a reducing agent) through a pot of glass and then bubbling oxygen, the combined effect of which is said to improve fining of the glass and to produce clear glass. That patent also describes the former practice of throwing carbonaceous objects such as potatoes or blocks of wood into a glass melt to temporarily increase the local fining action. Such an erratic technique is not considered suitable for a large scale, continuous glass melting operation.
U.S. Pat. No. 2,387,222 (Wright) discloses bubbling gas into molten glass in a melting and fining furnace for the purpose of controlling the circulation of glass in the furnace. It is disclosed that the gas may be oxidizing, reducing, or neutral. The purpose for using a reducing gas is stated to be "to retain certain colorants." There is no disclosure of which colorants are contemplated nor of an explanation as to how a colorant would be retained by making the glass more reducing.
U.S. Pat. No. 2,254,079 (McAlpine) proposes assisting the fining process by bubbling gases through molten glass in a fining zone of reduced depth. Carbon monoxide is one of the gases suggested as being usable in this process, although a troublesome brown coloration is described as requiring subsequent treatment with oxygen.